Being a sceptic and having experience with running computers with flaky graphic cards I haven't tried out 3D-accelerated desktops yet, but you should probably start by reading HOWTO: Compiz with NVIDIA Graphics Drivers. There's probably also a section about the same subject in the readme that came with your nvidia-drivers. (Installed in /usr/share/doc/nvidia-glx/README.txt.gz for me, but probably different places in other distros)